Bone implants are normally made of rigid material, mostly of titanium, which has been shown to have an affinity for bone tissue and which has an excellent biocompability. Bone implants often have a cylindrical, threaded shape and are screwed into bore-holes in the bone tissue which may be pre-tapped or not.
Under certain conditions titanium implants attain a close apposition with the bone tissue which sometimes is called osseointegration. Some factors determining the tissue response to a bone implant have been found to be the following: the biocompatibility of the implant material, the implant design, the implant surface, the status of the host bed, the surgical technique and the loading conditions. As far as the implant design is concerned, a review of the dental implant literature reveals that implants of a large number of different shapes have been used in the past. It appears as if new implant designs to a great extent have been introduced and evaluated on a trial and error basis. As the reason for an implant failure is multi-factorial, a good design may have been discarded due to for example an improper surgical technique or improper loading conditions. Titanium screw-shaped dental implants were used early in the 1960's, those implants do not appear to have been a success; possibly due to the reasons mentioned above.
Overloading has been identified as a main etiologic factor behind loss of dental implants today. If bone is subject to extreme stress it will be resorbed. Assuming that stress induced bone resorbtion is triggered off when the stresses reaches a certain level, an implant should have such a design that the maximum stresses arising in the bone, as a result of a certain load, is minimized.
Screw-shaped titanium dental implants dominate the market today. Several studies have addressed the relation between macroscopic design and holding power of screws in bone. By far most of them have been made within the orthopaedic discipline and have had an experimental approach. Pullout tests were carried out in the 1950's on dog femurs and tibias using vitallium bone screws with different thread profiles. It was observed that when pulling out a freshly inserted screw, the bone threads did not strip but the screw pulled out a small cone-shaped button of compact bone. Clinical experience shows that a bone plate and its screws are sometimes avulsed from bone. This avulsion is preceded by bone resorbtion. The opinion has been expressed that such a loss of holding power is caused by mechanical factors. Continuous compression of cancellous bone by screw threads has been shown to result in hypertrophy and realignment of the trabeculae in parallell with the force. It has also been claimed that cortical bone subjected to compression retains its integrity and is not resorbed.
The relevance of pull-out experiments can however be doubted. In a pull-out test acute fracture is provoked. Dental implants seldom fail by acute fracture of the supporting bone. On the contrary the fracture of the implant-bone interface is normally the end of a long process of marginal bone resorbtion. As mentioned above, the assumption that stress induced bone resorbtion is triggered off when the stresses reaches a certain level implies that an implant should be given such a design that the stress peaks arising in the bone are minimized.
It has been found that a bone implant being provided with threads or an oriented macro-roughness intended to transfer loads to the bone tissue and designed in accordance with the appended main claim minimizes the stress peaks in the surrounding bone tissue. Preferred embodiments are set forth in the dependent claims.